A thermal cut-out is a safety component, designed to break the circuit if the unit overheats for any reason, whether that is scale build-up, restricted flow, or a fault elsewhere in the unit. A cut-out that has tripped and stayed tripped, or one that trips repeatedly, is a common reason a shower stops working even though the heating element itself is fine. Trade buyers restocking cut-outs are usually replacing a component that has done exactly the job it was designed to do.
Neither option is universally better. A trade buyer fitting out a small annexe with one shower and one basin is usually better served by instantaneous. A buyer working on a house with a family's worth of daily hot water demand is usually better served by a cylinder sized correctly for the job. Getting the sizing wrong in either direction shows up fast, as lukewarm water, long waits, or an immersion element working harder than it should.
A flexible hose looks like the simplest part on a plumbing job, right up until the wrong one is fitted and something fails under pressure. Getting the spec right starts with bore size: the internal diameter of the hose has to match the fitting and the flow the job actually needs, not just whatever thread happens to fit at the tap end. Too small a bore restricts flow and pressure at the outlet; matching it correctly keeps performance consistent with the rest of the system.
See the details at trade plumbing supplies.
Plumb2U stocks flexible hoses across the bore sizes, lengths and pressure ratings trade buyers actually need, all carrying the approval marks that matter, at trade prices with next-day despatch. Picking the right connector first time, rather than the closest one to hand, is a small decision that avoids a much bigger callback later.
None of this is a guide to fitting a shower from scratch; it is a note on the specific parts that keep coming back across the working life of a unit already installed, and worth having in stock rather than ordering one at a time as each call comes in.